Extensive volcanism has occurred during Pliocene-Quaternary
in and around the Shimabara Peninsula, the back-arc region of
the Ryukyu Arc, Southwest Japan. Geological, geochronologic and
geochemical studies are performed to verify the relationship between
back-arc volcanism and regional tecotnism. Shimabara Peninsula
is located in western Kyushu Island, where no seismisity related
to the subducting Philippine Sea Plate is detected. Active Unzen
volcano situates in the middle of the peninsula, and is displaced
by the E-W trending Unzen graben, an active regional tectonic
graben. Volcanic products of Unzen subsided more than 1000 m beneath
the sea level inside the graben. Volcanism on the Shimabara Penisula
was most intensive and widely occurred between 2.5 Ma and 0.5
Ma before the birth of Unzen volcano. Monogenetic volcanoes of
olivine basalt and pyroxene andesite are major components. These
pre-Unzen volcanics form concaved major element variation trends
versus MgO suggesting that fractional crystallization is a governing
process of the magma differentiation. Basalts have significantly
lower HFS/LIL element ratios than those existing along the volcanic
front in the Ryukyu Arc, but show similar trace element abundances
to OIB-type alkalic basalts in the back-arc side of Southwest
Japan and eastern margin of E Asia. At 0.5 Ma, monogenetic volcanism
ceased and Unzen volcano started to grow in the central part of
the peninsula. Unzen volcanic rocks are hornblende andesites and
dacites, and are characterized by abundant large (>3mm) phenocrysts
of plagioclase and hornblende. Major element variations are on
a straight line between basaltic andesite and dacite suggesting
that magma mixing is a dominant process for the variation of Unzen
products. Unzen volcanics have higher Ba/Sr, Ba/Zr and Rb/Zr ratios
than pre-Unzen volcanics suggesting larger input of crustal materials.
Sr, Nd and Pb isotopic ratios of Pre-Unzen and Unzen volcanics
are closely similar suggesting the common source materials. They
are plotted on the mixing line between EM2 and N-MORB mantle components,
and fall in the region of OIB-type basalts of SW Japan but are
offset from those along the volcanic front.
Field geology, drill core analyses and K-Ar geochronlogy suggest
that formation of Unzen graben became significant after the birth
of Unzen volcano. Crustal thinning and consequent mantle upwelling
are synchronous with the graben formation, and this may have forced
the plume to ascend and magma formation to be centralized in the
middle of the Unzen graben at 0.5 Ma. A steady-state magma chamber
was formed to mix mafic and felsic magmas, and then have fed the
mixed magma to the surface, forming the composite Unzen volcano
in the back-arc side of the Ryukyu Arc.
･ Unzen volcano is located ~100km west from the volcanic front
of the Ryukyu Arc. No deep earthquakes occur beneath it. Many
E-W trending normal faults run in the middle of the Shimabara
Peninsula and displaced the central edifice of Unzen volcano constantly
(~2mm/y). The earliest products of Unzen volcano and the pre-Unzen
andesites, both are ~0.5 Ma, appear ~1000 m below sea level in
the central part of the Unzen graben.
･ In SW Kyushu, Neogene-Quaternary OIB-type alkalic basalts spread
sporadically along with calc-alkalic andesites and rhyolites.
Volcanism of Shimabara Peninsula is among such volcanic activities,
and is different from that along the volcanic front.
･ Volcanism started ca. 4 Ma, and spread on the whole peninsula
until 0.5 Ma when Unzen volcano started to grow in the center
of the peninsula.
･ Pre-Unzen volcanics are olivine basalts, pyroxene andesites
and pyroxene-hornblende andesites. All but earliest Unzen eruptives
are characterized by large hornblende and plagioclase phenocrysts.
･ Surface geology of Unzen volcano is strongly controlled by
the Unzen graben
･ Earlier Unzen products (>300 ka) appear only on NW and SW
outsite of the graben.
･ Thick lavas of 180-300 ka are distributed mostly inside of the
graben except for some lavas outfloored the graben.
･ Younger Unzen products (>150 ka) exist only in the eastern
half due to the existing of high ridges of older Unzen prohibiting
eruptives to flow westward.
･ Due to the ongoing subsidense of the Unzen graben, the products
of Unzen volcano has thickly accumulated inside the graben.
･ As fault activity has shifted toward the inner side of the graben
with time in general, and the base of Unzen volcano is deepest
(>-1000 m below sea level) in the centeral part of Unzen graben.
･ 3D geologic map and the air photo clearly show that Unzen
volcano has been disected and displaced by many E-W trending normal
faults.
･ Unzen volcano has continuously supplied lavas and pyroclastics
eastward during its whole growth history.
･ Two flank drillings (USDP-1 & 2) were made to recover the
eruption products inside the graben.
･ Conduit drilling (USDP-4) is planned to penetrate through the
magmatic conduit of 1990-95 eruption. The drilling will start
in Janurary 2003.
･ USDP-1 & 2 have penetrated the Unzen products and reached
to the pre-Unzen pyroxene andesite of 0.5 Ma at the depth of 680
m and 1180 m, respectively.
･ The earliest products of Unzen (~0.45 Ma) has smaller(< 2mm)
hornblende and plagioclase phenocrysts than the later products.
･ Pumice flow and fall deposits appear only in the earliest products
of Unzen.
･ Block and ash flows and associated lahar repeatedly reached
to the eastern fan of Unzen during its whole history.
･ Repeated supply of pyroclastic materials toward northeastern
slope in the short duration at 200 ka, and are thickly accumulated
agaist the northern graben wall.
･ Pre-Unzen basalts (diamonds) and andesites (circles) form
a concave variation trend with decreasing MgO content, suggesting
that fractional crystallization is a governing process for themagma
differentiation.
･ Unzen andesites and dacites (triangles), on the other hand,
plot on a straight line between basic andesite and dacite. This
suggests that magma mixing is dominant process for the variation
of Unzen products.
･ Pre-Unzen andesites to the north of Unzen (squares) form a different
variation trend from that of andesites to the south of Unzen (circles)
･Unzen andesites and dacites (triangles) have higher Cr contents
than Pre-Unzen andesites, suggesting that they are mixed with
Ni-rich basalt magmas.
･ Pre-Unzen basalts (diamonds) and andesites (circles and squares)
have similar Zr/Y and Ba/Sr ratios suggesting the common source
materials, while Unzen volcanics (triangles) have higher ratios
indicating contamination of crustal materials with higher Ba/Sr
and Zr/Y ratios.
･ Younger Unzen eruptives (yellow triangles), particularly those
with higher MgO, have lower Ba/Sr and Zr/Y ratios than older Unzen
products (green and pink triangles) suggesting more input of mantle-derived
components.
･ Spidergram of pre-Unzen basalts suggests that these basalts
have similar geochemical characteristic with surrounding basalts
in NW Kyushu (JB-1), but with lower LILE contents. Pre-Unzen and
Older Unzen andesites have similar but with some Ta deplition
and Rb, Th and Ba enrichments compared with pre-Unzen basalts.
･ Sr and Pb isotopic ratios of Unzen and Pre-Unzen volcanics
are offset from Quaternary volcanic rocks along the volcanic front
of the Ryukyu Arc, but within the range of the Miocene-Quaternary
alkalic basalts in NE Kyushu.
･ Pre-Unzen basalts and andesites are on the mixing line between
EM2 and N-MORB mantle components. Unzen andesites are within the
range of Pre-Unzen volcanics suggesting that all volcanics in
Shimabara Peninsula have the common source materials.
･ Volcanism drastically changed at 0.5
Ma in Shimabara peninsula.
･ Monogenetic volcanoes scattered in the whole Shimabara Peninsula,
and eruptives are on a fractional crystallization trend. Volcanism
was centralized in the middle of the peninsula where an active
graben was started to be formed, and Unzen stratovolcano has grown.
A clear magma-mixing trend is observed among Unzen products.
･ Hornblende-bearing pyroxene andesites erupted at 0.5 Ma have
transitional petrographic and geochemical characteristics between
Unzen and Pre-Unzen volcanics.
 |
Pre-Unzen
stage 0.5-2 Ma ･ Monogenetic volcanism of olivine basalt & pyroxene andesite across the whole Shimabara Peninsula ･ Weak development of magma chamber ･ Fractional crystallization dominant differentiation |
| Older
Unzen stage 0.15-0.5 Ma ･ Birth of Unzen startovolcano(hornblende andesite) in the middle of the Shimabara Peninsula ･ Development of Unzen graben (crustal thinning and the rise of Moho) ･ Formation of felsic magma by crust melting ･ Development of crustal magma chamber and effective magma mixing of mafic and felsic magmas ･ Explosive pumice eruption |
|
| Younger
Unzen stage 0-0.15 Ma ･ Conversion of volcanic and fault activities in the central part of the Shimabara Peninsula ･ Lava dome growth and dome-collapse pyroclastic flows ･ Repeated edifice failure |